Fuse link construction



Pa tented May 5, 1942 FUSE LINK CONSTRUCTION Ralph B. Pittman and Carroll H. Walsh,

Pine Blnil, Ark.

Application November 12, 1940, Serial No. 365,286

2 Claims.

.This invention relates generally to protective devices for electrical circuits, and particularly to fuses used to limit to a predetermined quantity the passage of electrical energy through connected circuits.

ing, without fusing, a much higher current for fractional parts of a second.

It is well known that many existing types of fuses possess in some degree the inverse timecurrent characteristic stated in the above object. It is also well known that such fuses, if capable of operation on small currents, of the order of /2 to 5 amperes, even in long periods of time,

lack the surge capacity to carry for short times the high currents, of the order of hundreds of amperes, which are often imposed on overhead distribution conductors under lightning storm conditions. Such cur-rent surges are in general harmless to connected apparatus, but capable of causing fuse outages, with resulting interruption of the electricity supply.

Another object is the provision of a simple, effective and inexpensive device for the purpose described.

With these and other objects in view which will appear as the description proceeds, our invention resides in the novel construction, combination and arrangement of parts as hereinafter described, and the scope of the invention will be defined in the appended claims.

In the drawing, Fig. l is an elevational view of the upper portion of the fuse link; Fig. 2 is an elevational view, in section, taken along the line 11-11 of Fig. 1; Fig. 3 is a section. taken usual type of fuse link housing. An upper plug l2, of some insulating material, such as fiber, is fitted into the upper end of the container Ill, and may be secured in place by crimping a portion of the container tightly against the plug around its outer surface.

A stepped recess extends downwardly through the center portion of the plug l2, coaxially with the container II, and the button or head ll of the metal supporting and connecting member it engages the step, the member I6 extending coaxially into the tube In for a short distance.

Within an axially upwardly extending opening of the member I6 is secured, by pressing the respective elements together, one end of the fusible element It, and one end of the helically formed heater coil l3. The latter two members may be formed from a nickel-chromium-iron alloy wire, this material being of a high specific electrical resistance as compared to usual materials employed for electrical conductors, such as copper or aluminum. The fusible element ll preferably extends downwardly within the container Ill from its point of attachment to the member l6, along the axis of the container l0 and the helical coil Hi, the lower end of the latter being mechanically secured and electrically connected to the container l0 by crimping at IS.

The lower end of the fusible element It emerges from the helical coil I3 within the container Ill, and is mechanically secured and electrlcally connected, by crimping the respective members, to the conducting member IS, the latter 4 extending coaxially endwise from the container I 0, and loosely through a centrally located opening in the lower insulating plug IS.

A stop button 20 is crimped to the conducting member I8 in spaced relation with the outer surface of the plug l9, and arranged to engage an insulating washer M. A compressed coil spring 22 engages at its respective ends the plug l9 and the washer 2|, the arrangement being such that the spring constantly urges the conducting member l8 endwise from the tube i0, thus continously tensioning the fusible element I4.

The electrical circuit through the fuse link is from the flanged portion ll of the container iii to its connection with the lower end of the helix I3 at i5, thence upwardly through the convolutions of the helix l3 to the conducting member l6, and thence downwardly through the fusible element H to the conducting member ll.

It may be noted that the heater coil l3, through which the fusible element H longitudinally extends, is not subjected to tension from the spring 22; also that the fusible element I4 is continuously tensioned by the spring 22.

The helical coil i3 and the fusible element ll, being electrically in series, and positioned as described, provide an arrangement such that the fusible element [4 operates at a higher temperature than the heater coil l3 in response to currents through the series arrangement which continue sufliciently long to obtain heating,.by radiation, of the fusible element ll by the heater coil l8. For very short-time impulses or surges I rt Temperature rise where temperature rise is that caused by the passage of a current I for a time t through a conductor having a resistance r, a mass m, and

A a specific heat 3.

Noting that kl X r;

where k is a. constant depending upon material I is length a is cross-sectional area,

and substituting (2)in (1), it will be seen that temperature rise caused by a current I is directly proportional to the time the current flows, and inversely proportional to the square of the crosssectional area, since the mass of a conductor is a direct function of both its length and area.

As t approaches zero-a condition attending the passage of surge currents through the fusethe time is so short that the heat transmitted from the coil l3 to the fusible element I4 is negligible, and the area of the fusible element is the predominating consideration. To illustrate, substantially the same surge. current will fuse identical fusible elements one of which is submerged in water, and the other of which is surrounded by a heater coil.

However, if a sumciently large cross-sectional area is used to provide the desired surge capacity in a simple longitudinal fusible element, too much dynamic or relatively long-enduring current may be required to fuse it to make the device suitable for its intended use, since what is desired is a fuse having high surge current carrying capacity and low dynamic current carrying capacity.

To provide the latter characteristic, we arrange the fusible element It so that it may be slowly heated by the coil l3 to fusion upon the occurrence of a predetermined current which, unlike a current of the character of surge-current, per-- sists for a predetermined time. The construction is thus one which carries unusually large currents for short times of the order of a few micro-seconds, but will fuse at unusually small currents for times of the order of a few minutes.

'While' this invention has been described in considerable detail, it is to be understood that such description is intended as illustrative rather than limiting, as the invention may be variously embodied, and is to be interpreted as claimed.

We claim, as our invention:

1. A fuse link construction comprising a fusible element and a helically formed heater coil through which said fusible element extends, a tubular container of conducting material containing said helical coil and said fusible element, spring means associated with said fusible element and positioned externally at one end of said container, said spring means, urging said fusible element in an endwise direction to withdrawal from said container, conducting means at the other end of said container electrically connecting one end of said helically formed heater coil to one end of said fusible element, and means including an insulator interposed, between said conducting means and said container for fastening the joined ends of said coil and fusible element to the other end of said container.

2. A fuse link construction comprising a fusible.

positioned stepped recess, conducting means having a first portion thereof resting on the step of said recess and positioned within the container beyond the outer surface of said flange land a second portion extending through said recess within said container, means mechanically fastening and electrically connecting one end of said coil and the adjacent end of said fusible element to said secondportion, the other end of said coil being directly fastened and electrically connected to said container and the 

